Nuclear spin resonance (NMR) apparatus, such as NMR spectrometers and NMR tomography machines, require strong magnetic fields which are typically generated by superconducting magnet coils. The superconducting magnet coils must be operated at a cryogenic temperature. For this, the magnet coils are typically arranged in a cryostat. The cryostat comprises an evacuated container (“vacuum container”), in which an object to be cooled is arranged, often further surrounded by a radiation shield. The object to be cooled may be the magnet coil itself (“cryofree” system), or also a cryocontainer, in which a cryogenic liquid (such as liquid helium) and the magnet coil are arranged.
The object to be cooled is generally cooled by an active cooling system, usually comprising a pulse tube refrigerator or a Gifford-MacMahon cooler. Active cooling systems reduce the consumption of costly liquid helium, increase the availability of the NMR apparatus, and may also contribute to reducing the structural height. The active cooling system may be a single-stage or a multiple-stage configuration. In multiple-stage systems, usually a warmer cold stage cools the thermal radiation shield and a colder cold stage chills the object to be cooled.
Upon malfunctioning of the active cooling system, the superconducting magnet coil or the superconducting magnet coils (“superconducting magnet coil system”) should be able to remain below the critical temperature until a repair of the active cooling system can be undertaken. A loss of the superconducting state due to a warm-up may result in the destruction of the superconducting magnet coil system; but also, a renewed cooling of the superconducting magnet coil system would at least entail substantial expense.
In the most common designs of cryostats with active cooling, cf. US 2007/089432 A, the cooling arm of a cooling head protrudes into a neck tube of a vacuum container. The neck tube is open towards a cryocontainer, in which a superconducting magnet coil is arranged in liquid helium. At the lowermost cooling stage of the cooling arm, helium recondenses and drips back into the cryocontainer. Similar cryostats are known from US 2010/298148 A, US 2007/022761A, German Patent No. DE 10 2004 012 416 B4 or US 2007/051115 A.
In order to afford the longest possible time between a malfunctioning of an active cooling system and a service intervention, the thermal masses (i.e., the mass multiplied by the specific heat capacity) in the cryostat, such as a radiation shield or a cryocontainer including cryogenic liquid, may be chosen to be large, but this increases the structural height and the overall weight of the cryostat. Likewise, in cryostats with a cryocontainer, it is possible to replenish externally procured liquid helium in order to replace evaporated helium; yet this is very costly.
According to U.S. Pat. No. 8,950,194 B2, a portion of the evaporating gas from the cryocontainer may be conducted along the cooler when the cooler is shut off, for example during transport, and thereby reduce the thermal burden on the cooling arm.
According to German Patent No. DE 10 2014 218 773 A1, a cavity in a cryostat between the inside of the neck tube and the cooling arm of a cooling head may be filled with a gas, such as helium. In normal operation, the lowermost cooling stage of the cooling arm is near the object being cooled; for example, a good thermal coupling may be produced by a contact between the object to be cooled and the lowermost cooling stage for a small quantity of liquid helium in the cavity. Upon loss of the cooling, the gas pressure in the cavity rises as a result of warming; liquid helium in the cavity evaporates. The movably mounted cooling head is moved by the rising gas pressure in the cavity in a direction away from the object to be cooled, which decreases the thermal coupling between the cooling arm and the object to be cooled.
With this cryostat, the thermal load can be decreased by a cooling arm upon loss of the active cooling, but the design expense is relatively large due to the movable suspension of the cold head. Furthermore, due to the high gas pressure in the cavity, a thermal coupling persists that is not insignificant.